At Port Meadow alone, on the outskirts of Oxford, they estimate 27.5 tonnes of ammonium a year find their way from landfill into the River Thames. The researchers say it could be happening at thousands of sites around the UK.

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In water, ammonium breaks down into nitrogen. The extra nitrogen can trigger excessive plant growth and decay, damaging water quality and starving fish and other aquatic organisms of the oxygen they need to survive.

Scientists are most worried about so-called blue-green algal blooms, which can produce toxins capable of killing wild animals, livestock and domestic pets. In people, they can cause skin rashes, nausea, stomach pains, headaches and fever.

'We've been getting rid of waste for an awful long time,' explains Dr Daren Gooddy, of NERC's British Geological Survey, who led the study. 'Since Victorian times, we've been putting it into landfill and ad-hoc waste dumps on the edge of our towns and cities, often on the fringes of floodplains.'